Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. Pixels and sub-pixels are not distinguished herein; all light-emitting elements are called pixels. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode displays. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Light-emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 to Tang et al. shows an organic LED color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
LED devices can include a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. Alternatively, one can employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 by Cok. It is also known to employ a white sub-pixel that does not include a color filter, for example, as taught in U.S. Pat. No. 6,919,681 by Cok et al. A design employing an unpatterned white emitter has been proposed together with a four-color pixel including red, green, and blue color filters and sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device (see, e.g. U.S. Pat. No. 7,230,594 to Miller, et al).
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In an active-matrix device, control elements are distributed over the flat-panel substrate. Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the brightness of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode.
Prior-art active-matrix control elements typically include thin-film semiconductor materials, such as silicon, formed into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors made from amorphous or polycrystalline silicon are relatively larger and have lower performance than conventional transistors made from crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity that results in perceptible non-uniformity in a display employing such materials. While improvements in manufacturing and materials processes are made, the manufacturing processes are expensive and thin-film device performance continues to be lower than the performance of crystalline silicon devices.
Matsumura et al discuss crystalline silicon substrates used with LCD displays in U.S. Patent Application Publication No. 2006/0055864. Matsumura describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. However, there is no teaching of improving the aperture ratio of a display, reducing the cost of such pixel-control devices in cooperation with a display device, or testing the pixel-control devices.
Electronic device testing is well known in the prior art. For example, U.S. Pat. No. 6,028,441 describes self-testing routines in an LED display device by monitoring current use by the LEDs. U.S. Pat. No. 5,369,357 describes an optically operated test structure for a CCD imager for testing the modulation transfer function for the CCD. Electrical testing methods for OLED devices are described in U.S. Patent Application Publication 2007/0046581 and in U.S. Pat. No. 6,995,519.
Yields are important in manufacturing low-cost flat-panel displays. It is important, therefore, that any flaws in the manufacturing process be detected as early as possible so as to repair the flaws or discard the flawed devices without incurring any further manufacturing expense. In the prior art, flat-panel displays are tested after manufacture and repaired, if necessary. By testing displays during the manufacturing process, the cost of repair is reduced and the manufacturing yields improved. It is also important to test devices in an efficient way. In particular, displays with many pixels (e.g. high-definition televisions) can take a long time to sequentially test each pixel. Therefore, test methods that can be implemented quickly are useful in the manufacturing process.
There is a need, therefore, for improving the performance of active-matrix light-emissive displays and testing such displays in an efficient and effective manner, in a short period of time, during or after manufacture, in order to improve the manufacturing yield of the active-matrix light-emissive displays.